Method of selecting a preferred difference image

ABSTRACT

In a radiographic digital subtraction system utilizing a contrast agent for enhancing a difficult-to-image anatomic region of interest, a method of selecting a superior difference image. The difference image selected represents the differential between a pair of digitized radiographic images selected from a series of such images acquired before and after the contrast appears in the region of interest during a dynamic imaging study.

TECHNICAL FIELD

This invention relates to dynamic image enhancement of low contrastanatomic regions utilizing a contrast material introduced into a patientduring the acquisition of a series of radiographic images. Inparticular, the invention relates to a method of selecting a preferreddifference image representing the differential between a reference ormask image acquired before the contrast material takes effect and alater acquired post-contrast image.

BACKGROUND ART

In a copending application Ser. No. 138,400 of Robert Henry McCarthy,filed Apr. 8, 1980, for DYNAMIC IMAGE ENHANCEMENT AND APPARATUS THEREFORand assigned to the assignee of the present invention, there isdisclosed a digital subtraction radiographic system for imageenhancement of low contrast objects utilizing venous injection of acontrast material to visualize a difficult-to-image anatomic region ofinterest. In the digital subtraction system, disclosed in the copendingapplication, a radiographic digitized composite mask image of a regionof interest of a patient is acquired and stored in a first digitalmemory. During the clinical procedure, a quantity of contrast material,preferably an X-ray opaque dye, is introduced into the patient.Depending on several factors including the type and quantity of contrastmaterial, physical characteristics of the patient, and the distancebetween the remote vein and the region of interest, such as a carotidartery, the time for the contrast material to take effect varies,typically between approximately 10 and 15 seconds. After the contrastmaterial takes effect, a second radiographic image substantially of thesame region of interest as the reference image is acquired and stored ina digitized form in a second digital memory. Alternative techniques aredisclosed for subtractively combining the two digitized images to cancelthe overlapping background, thereby enhancing the region of interestwhich shows the contrast material. The resultant subtracted image, knownas a difference or differential image, devoid of a significant amount ofnon-useful background information, permits superior clinical evaluationof the difficult-to-image region.

A clinical investigator about to perform a diagnostic study involvingthe acquisition of a series of radiographic images generally will notknow beforehand the length of time for the contrast material to takeeffect in the anatomic region of interest after injection since thereare so many factors in this determination. Thus, an image acquired earlyin the study, typically the first such image, will be designated thereference or mask image. Each subsequently acquired image then becomes acandidate for the post contrast image to be subtracted from the mask toyield the desired difference image.

The ideal difference image is generated from the pair that permits thegreatest amount of cancellation of background or non-useful informationand yields the most information in the region of interest. The firstcondition requires a high degree of overlap of the two images which isdetermined principally by the degree of movement in the imaged region.The second condition is a function of the concentration of contrastmaterial in the region of interest and this is a function of time.Patient movement, both voluntary and involuntary, has posed a sufficientclinical problem that the investigator would prefer to use as the mask,an image which comes as close in time as possible prior to theappearance of a substantial quantity of contrast material in the regionof interest to significantly minimize motion between the time of theacquisition of the mask and the acquisition of the post contrast image.Hence, the resultant difference would provide the desired high contrastin the region of interest with maximum overlap and hence cancellation ofthe non-useful background.

For any given mask, the system disclosed in the aforementioned copendingapplication, permits generating a series of difference images bysubtracting from the mask, independently and sequentially, any otherimage, i.e. a post contrast image, and viewing the difference. Though atrained physician can typically and rather expeditiously select the bestdifference image once a mask is preselected, the result will notnecessarily yield the best difference image available in the study sincethe original choice of mask may not have been the best since the choicewas at best an informed guess. Moreover, even if this process weregenerally acceptable when performed by an experienced investigator, thetechnique is not likely to consistently yield acceptable clinicalresults when operated on a grand scale.

The procedure of introducing contrast material into the object studiedis complex and must be coordinated with the operation of an X-ray systemand data collection system. To require concurrent analysis and decisionmaking on the quality of images acquiring burdens the operators and thesystem. The present invention permits the data to be collected over along enough time interval to insure the collection of the best mask anddata (contrast) image. Then, as a leisurely post-processing operation,one can select the best pair of images to subtractively combine.Furthermore, since the contrast material does not necessarily reach allparts of the image at one time, it is doubtful whether the a prioriselection of a single mask would be adequate to an analysis of theentire image.

DISCLOSURE OF THE INVENTION

Applicants have discovered a "remasking" technique for consistentlygenerating a preferred difference image from a series of images acquiredduring a dynamic imaging study. The improved method requires a minimumof training and a minimum of operator time.

The method is applied in a radiographic, digital subtraction system forimaging a low contrast anatomic region of interest of a patient byacquiring a plurality of mask or reference images of the region ofinterest, introducing into the patient a contrast material suitable forenhancement of a clinically relevant region, acquiring a plurality ofpost contrast images of the anatomic region and generating a pluralityof difference images in which overlapping portions are significantlyeliminated by digitally subtractively combining a plurality of pairs ofreference and post contrast images.

The improved method comprises storing the plurality of reference andpost contrast images acquired during the study. As a practical matter,the series of images need not even be designated as pre or postcontrast, but merely stored. The method further comprises selecting asample difference image which shows contrast material in the clinicallyrelevant region of anatomic interest. The sample difference image isviewed on a video display and a first area from the region showingcontrast material is identified. Similarly, a second area is identifiedfrom the background portion of the image, the background portion showingat most a minimal amount of contrast material. Each area from eitherregion is recorded, preferably electro-optically, such as by a light penor joystick. The stored reference and post contrast images are thensearched to select those pairs which when subtractively combined yieldthe best difference images in terms of maximum overlap of background.Finally, from the difference images so selected, the one which exhibitsthe greatest activity in the contrast region is selected as a preferreddifference image for clinical analysis.

BRIEF DESCRIPTION OF THE DRAWINGS

The single FIGURE is a schematic flow diagram showing steps of thepresent invention.

BEST MODE OF CARRYING OUT THE INVENTION

Referring to the drawing, a series of radiographic digitized referenceor mask images are chronologically acquired and serially stored in aMemory A. These images visually represent a clinically relevant regionof interest of a patient acquired before a contrast agent introducedinto the patient takes effect. After the contrast agent or materialtakes effect another series of digitized images are chronologicallyacquired and serially stored in Memory B. The total study per patientincludes a series of n composite frames where n is typicallyapproximately 30. Of the n frames in a study the pre-contrast portion isdesignated x₁ . . . x_(i) and the post-contrast portion is designatedx_(j) . . . x_(n).

The operator of a multi-memory radiographic digital subtraction systemto which reference has been made hereinabove under the Background Artportion of this application, may be remote computer control, view asample difference image generated by subtracting a post-contrast image(taken from the group x_(j) . . . x_(n)) from a pre-contrast orreference image (taken from the group x₁ . . . x_(i)). While allpossible difference images (x₁ -x_(j)) . . . (x_(i) -x_(n)) canconceivably be generated and stored, such a procedure would beinefficient and costly as well as requiring an undue length of time fora clinician to review. As an alternative, a sample difference image, forexample, x₁ -x_(j), is generated and displayed on a video screen forviewing. The sample difference image x_(l) -x_(j) must be one thatincludes both a portion of the clinically relevant region of interestshowing contrast material as indicated by the diagonally cross-hatchedportion of the illustrated sample difference image on the drawing andthe background region showing little or no contrast material indicatedby the vertical lines on the drawing representative of the motionartifact. On such a sample difference image x₁ -x_(j) an area A.sub. 1is selected from the background portion and an area A₂ is selected fromthe clinically relevant region of interest showing contrast material.Enclosed regions A₁ and A₂ represent some random number of addresses ofthe, for example, 256×256 pixel array of the video screen (not shown)that is provided in a radiographic digital subtraction system. Theselected areas A₁ and A₂ are electro-optically recorded, preferably by alight pen or joystick. Once these regions A₁ and A₂ are identified andrecorded all address points falling therewithin can be quickly searchedon any stored image.

During a study all of the images n are stored on a mass storage device,such as on tape or disk. The next step in the preferred method is tosearch through the n images on all points falling within areas A₁ and A₂to select a predetermined number of pairs y of images (one from thegroup x₁ . . . x_(i) and one from the group x_(j) . . . x_(n)) whichyield the best overlap in region A₁. The best overlap is provided bythose pairs which when subtractively combined yield difference images inwhich region A₁ has the lowest composite values denoting the least levelof motion artifact. This is done, for example, by summing the values ofthe differences of each address falling within the region A₁ andselecting the lowest cumulative totals, or by finding the meandifference or some other convenient index. The predetermined number ofsuch pairs y may, for example, be 10. After this search and select stepthe y difference images generated are searched to determine which onehas the greatest activity, i.e. showing the most contrast material inregion A₂, as indicated by the highest cumulative values generated inregion A₂ after subtraction. The difference image which is selected inthis step is a preferred difference image.

Alternatively, the post contrast image showing the most contrastmaterial in region A₂ can be pre-selected. Region A₁ of thatpre-selected image can be compared with each of the remaining storedimages to select the one which when subtractively combined with thepre-selected post-contrast image will yield the greatest cancellation inthe background region.

We claim:
 1. A method of selecting one or more x-ray difference imagesfrom a group of x-ray images, which comprises the steps of:(a) obtainingfirst and second pluralities of x-ray images of a patient or subject,each x-ray image being obtained at a different time; (b) obtaining asample difference image representing the difference between one of theimages of the first plurality of images and one of the images of thesecond plurality of images; (c) selecting a region of the sampledifference image, the region being less than the entire area of thesample difference image; (d) pairing images selected from the firstplurality of images with images selected from the second plurality ofimages; (e) obtaining a plurality of difference images each representingthe difference between the images of one of the pairs of images; (f)searching regions of the difference images, the searched regionscorresponding, respectively, to the selected region of the sampledifference image; and (g) selecting one or more difference images, theselected difference images having searched regions that satisfy apredetermined condition or index.
 2. A method according to claim 1,wherein the region of the sample difference image is selected at a timesubsequent to the time at which the first of the images is obtained. 3.A method according to claim 1, wherein the images are digital videoimages each formed by a matrix of pixels.
 4. A method according to claim3, wherein the pixels of each image are stored on a mass storage device,the searching of regions of the images being accomplished by processingof pixel data assigned to computer memory address locations related tothe assigned computer memory addresses of pixels in the selected regionof the sample difference image.
 5. A method according to claim 4 whereinthe pixel values in the searched regions are mathematically processed toobtain a result which is compared with a reference value based uponpixel values in the selected region of the sample difference image.
 6. Amethod according to claim 1, wherein a second region of the sampledifference image is selected, the second region being less than theentire area of the sample difference image, the first-mentioned regionhaving substantially different contrast characteristics than the secondregion.
 7. A method according to claim 1, which further includes thesteps of:(h) selecting a second region of the sample difference image,the second region being less than the entire area of the sampledifference image and different than the first-mentioned selected region;(i) in the difference images selected according to step (g) of claim 9,searching such selected difference images in regions thereofcorresponding, respectively, to the second selected region of the sampledifference image; and (j) selecting one or more difference images, theselected difference images having searched regions, corresponding to thesecond selected region of the sample difference image, that satisfy apredetermined condition or index.
 8. A method according to claim 7,wherein one of the selected regions of the sample difference image is aregion having a high degree of contrast with the background of thesample difference image and wherein the other of the selected regions ofthe sample difference image is a region having a low degree of contrastwith the background of the sample difference image.